INITIATION OF ANTI-RETROVIRAL THERAPY AT 48 HOURS POST SIV INFECTION POTENTLY SUPPRESSES ACUTE PHASE VIREMIA BUT FAILS TO PREVENT DISEASE We have investigated whether a 28-day course of potent ART, initiated at a time (48h p.i.) following SIV inoculation when acquisition of a viral infection was virtually assured, would sufficiently sensitize the immune system and result in controlled virus replication when treatment was stopped. Administration of Tenofovir 48h post SIV inoculation to six Mamu-A*01 negative rhesus macaques did, in fact, potently suppress virus replication in all of the treated rhesus macaques but plasma viral RNA rapidly became detectable in all six animals following its cessation. Unexpectedly, the viral set-points in the treated monkeys became established at two distinct levels. Three controller macaques had chronic phase virus loads in the 1 x 10e3 RNA copies/ml range whereas three non-controller animals had set-points of 2 to 8 x 10e5 RNA copies/ml. All of the non-controller monkeys have died with symptoms of immunodeficiency by week 60 post-infection, whereas two of the three controller animals are alive at week 80. Interestingly, the three controller macaques each carried MHC class I alleles previously reported to confer protection against SIV and two of these animals generated CTL escape viral variants during the course of their infections. WHAT LEVEL OF CIRCULATING NEUTRALIZING ANTIBODIES CONFER PROTECTION TO MACQUES FROM A SUBSEQUENT SHIV CHALLENGE? The development of an effective vaccine against the human immunodeficiency virus (HIV) is critical for controlling the acquired immunodeficiency syndrome (AIDS) epidemic. A variety of assays have been employed to measure the anti-viral cellular and humoral responses elicited by these vaccines to delineate correlates of protection. The results from studies involving both SIVs and SHIVs have shown that pre-challenge cellular responses alone do not prevent virus acquisition, although they can reduce the peak and set point levels of viremia. The contribution of vaccine-induced neutralizing antibodies (NAbs) in controlling virus infection has been more difficult to evaluate. Measurements of the level and breadth of NAb responses in samples collected from vaccinees, infected individuals, or HIV surrogate animal models have relied on a variety of anti-viral neutralization assays. Early assays relied on the control of spreading virus infections, requiring that cultures be maintained for extended periods of time for the anti-viral neutralization effects to become evident. The development and use of the TZM-bl cell based assay has greatly simplified measurements and reduced inter-laboratory variability in monitoring anti-HIV 1 NAbs. Despite the plethora of data generated with the TZM-bl system, the assay has not yet been used to determine virus neutralization titers that confer protection to individuals or animals exposed to virus. We previously administered purified IgG containing high titered polyclonal NAbs against SHIVDH12 to 21 pigtailed macaques and determined the NAb titer (1:38) in the plasma required to protect 99% of animals against an intravenous (IV) challenge of 75 TCID50 of virus ( ). This protective titer was derived from a 14-day MT4 cell assay, which measures the complete inhibition (EC100) of SHIVDH12 replication in cell cultures at the limiting dilution end-point. The TZM-bl system was used to measure EC50 neutralizing titers in several of the same macaque plasma samples and augmented probit regression was employed to determine EC50 titers conferring various levels of protection. When psudotyped virus was used in the TZM-bl assay, it was determined that 99% protection would require an EC50 titer of 1:4467 (not 1:38 as measured in the MT4 assay). Because it is likely that contributions from other arms of the immune system would contribute to vaccine-induced control, a range of protective EC50 NAb titers for this data set were also derived: 99% / 1:4467;90% / 1:1175;80% / 1:676;50% / 1:234;and 33% / 1:141. This range of titers may be more informative for evaluating the protective value of NAb activities determined on patient samples using the TZM bl assay.